U.S. Patent Publication No. US 2005/0193772, entitled “Closed Cassette and Method for Heating Treating Glass Sheets,” (the '772 publication) discloses a cassette for use in heat treating glass sheets. As explained in the '772 publication, manufacturers of glass sheets often heat treat glass sheets prior to shipping them to customers so that the sheets do not shrink or shrink very little when used in the customers' processes. Such heat treatments are known as “pre-shrinking,” “pre-compacting,” or simply “compacting.” These heat treatments differ from annealing in that they are performed at lower temperatures, e.g., temperatures below the strain point of the glass making up the sheets.
As one example of the need for pre-shrinking, the glass substrates used in the manufacture of liquid crystal displays, especially those employing poly-Si technology, are exposed to relatively high temperatures during the display manufacturing process. If not pre-shrunk, the substrates can undergo shape changes which are large enough to adversely impact the quality of the finished display. By pre-shrinking the glass sheets which form the substrates, the occurrence of this problem can be significantly reduced.
In addition to exhibiting low levels of residual compaction in customers' processes, the glass substrates used in the manufacture of liquid crystal displays, as well as in other electronic applications, need to satisfy a variety of additional demanding criteria. For example, the surfaces of the substrates need to be free of glass chips and particles which can cause defects in the finished displays. Also, the substrates need to have substantially flat surfaces with low levels of warp and other surface distortions. Such flat surfaces are, for example, important in liquid crystal displays because the operation of the display depends on the local thickness of the liquid crystal material used in the display.
Accordingly, the pre-shrinking of glass sheets for use in electronic applications including liquid crystal display applications represents a challenging problem because the heat treatment used to achieve the pre-shrinking must not lead to other problems. In particular, the pre-shrinking needs to be performed without creating glass chips which can contaminant the surfaces of the glass and without distorting the flatness of the sheets' surfaces through spatially non-uniform heating and/or cooling patterns.
The '772 publication discloses a cassette and heat treatment process which address various aspects of the challenges of the pre-shrinking problem. In accordance with the '772 publication, a plurality of glass sheets are held in a vertical orientation in a plurality of what is referred to in the '772 publication as “enclosed sections” of a “cassette” (see reference numbers 406 and 400 in the '772 publication). The “enclosed sections” (referred to in the present application as “containers”) include horizontal and vertical support bars (see reference numbers 424 and 428 in the '772 publication).
The present invention is directed to providing improved containers and support systems for use in heat treating glass sheets. It is also directed to improved methods for performing heat treatments, which methods preferably employ the improved containers and support systems of the apparatus aspects of the invention but can employ other equipment if desired. The heat treatments are preferably employed to achieve pre-shrinking of glass sheets, but can be used for other purposes, e.g., annealing. The apparatus and methods of the invention are preferably employed in the overall system and methods of the '772 publication, the contents of which are incorporated herein by reference in their entirety, but can be employed in connection with other systems and methods if desired.